The invention relates to a device for accurate position control of a force-transmitting system comprising at least one solid-state piezo actuator. The European Patent EP 0602492 discloses a cluster mill where piezo actuators are used to control the roll bending of a back-up roll. Apart from this specific application to a cluster mill, the use of piezo actuators is also conceivable for a variety of other devices where accurate position control of a force-transmitting system is desirable. In the present application, the invention will be described specifically with reference to the use in cold- or hot-rolling of metal strip. The invention is not, however, limited to this field of application.
As a rule, the rolling force is applied by means of hydraulic systems or electrically driven threaded spindles. A drawback of these systems resides in the fact that a relatively long time is required to achieve a specific deflection of the positioning system and that the positioning accuracy is relatively low. The primary purpose of using piezo actuators is to be capable of rapidly achieving a specific desired deflection of the position control system. Another objective is to be capable of more accurately achieving a predetermined desired deflection of the position control system. Further advantages of the use of a piezo actuator are generally known.
As well as advantages, the use of piezo actuators also clearly carries drawbacks, however. One drawback resides in the fact that piezo elements are able to withstand only a limited axial load. Whilst there is still considerable uncertainty concerning this issue, it is nevertheless assumed that an axial load of a circular piezo element having a diameter of about 50 mm should not exceed 75 kN. Particularly where thickness control in the rolling of steel is concerned, unexpected peaks in the rolling force may occur, which could lead to serious damage to the piezo elements.
Another problem in using piezo elements resides in the fact that piezo elements must be subjected to very purely axial loads and are very sensitive to damage in the event of deviations from the purely axial load.
It is therefore an object of the invention to provide a device which allows all the desired advantages of the use of piezo actuators to be utilized while avoiding the consequences of the drawbacks attendant upon said use. Other advantages resulting from the use of the novel device, while drawbacks are avoided, are clarified hereinafter.
The device according to the invention is then, firstly, characterized in that the piezo actuator comprises a metal block which is positioned between end plates and in the direction of the force has at least one duct into each of which a stacked piezo element fits, the height of the block being shorter than the length of the piezo elements and the difference in height corresponding to the shortening of the piezo elements under the maximum permissible axial load acting thereon, and in that the piezo actuator is incorporated transversely to the direction of force, without play, in such a way into an element of the force-transmitting system it is possible to ensure that only a force in the axial direction of the ducts can be transmitted to the piezo elements. It should be noted in this context that a stacked piezo element is understood to be a number of loose piezo elements which are stacked to form a rod-shaped body and are formed into a unit, for example by means of gluing. As a result of the stacked piezo elements being placed into a duct, it can be positioned in such a way that the direction in which it may be loaded accurately matches the force direction of the system. If the transmitted force threatens to exceed the permissible maximum, the stacked piezo elements are compressed to such an extent that they no longer project beyond the block. From then onwards, the block will absorb the axial force and in so doing protect the piezo elements owing to the fact that the block has a very much higher stiffness than the piezo elements. In the process, the block, as it were, fulfils the function of a blocking spring.
In this arrangement, any transverse force is absorbed by this element without a transverse force being transmitted to the piezo elements.
It was found that fabrication of piezo elements and the stacking thereof may lead to slight differences in length between them. The differences in length can be reduced to less than 50 xcexcm with the aid of shims. For the purpose of good force transmission by the actuator as a whole, at least one of the end plates can be constructed as a thrust bearing.
Cluster mills can be subject to differences in temperature, while cooling liquids are often used extensively. The operation of piezo elements is to some extent temperature-dependent, whilst they are also, regarding their service life, highly sensitive to excessive exposure to moisture. Moreover, the functioning of the metal block as a blocking spring depends greatly on its temperature, the reason for this being that the expansion coefficient of steel is larger, by about a factor of 3, than that of the ceramic piezo elements. It was found that all these drawbacks can be overcome, according to the invention, if the ducts within which the stacked piezo elements are located, are connected to a source of a gas for controlling the temperature and the humidity level within the ducts. As a rule, air will be used here. By supplying air under some excess pressure it has proven possible to avoid the ingress of splashed fluid as far as the piezo elements.
In the most ideal case, the rolling force can be completely absorbed by means of the piezo elements, allowing overloading to be avoided with the aid of the block within which these elements are located. Piezo elements are expensive, however, and take up space. If limited space is available for putting piezo elements into place, it is advantageous, according to the invention, for the piezo actuator to be also coupled to at least one bias spring, the bias force not being strongly dependent on the travel of the piezo actuator. By allowing part of the rolling force to be absorbed by one or more bias springs, the volume required for putting piezo elements into place can be kept smaller, costs also being limited at the same time. The use of bias springs having relatively low stiffness ensures that the bias force does not greatly depend on the travel of the piezo actuator, as a result of which the performance of the piezos is not greatly reduced by part of their function being allowed to be assumed by bias springs. As the bias springs do not require more than a very small travel, it is advisable for them to take the form of hydraulic membrane actuators. A packingless and maintenance-free design is thus obtained. These bias springs may also be given low stiffness by being coupled, for example, to a large buffer vessel for the hydraulic fluid.
A further aspect of the invention is that the piezo actuators and/or the bias springs are provided with height sensors. In this arrangement, signals coming from these height sensors can be added to the control system of the electrical actuation of the piezo elements and/or of the hydraulic actuation of the bias springs, this arrangement allowing the bias springs to be optimally activated in the range of the position control in which the piezo actuators function. The signal coming from the height sensors in the piezo actuators can be used directly for controlling the position controller.
The new device, in particular, proved to have good utility if the piezo actuator is incorporated in a bearing housing of a cluster mill.